Device Threshold Voltage
Scaling down the supply voltage allows for compensating speed loss by adjusting the device’s threshold voltage (Vt). The reduction in Vt, achieved through changes in substrate and channel dopant concentrations, enables scaling down the supply voltage without sacrificing speed. However, this scaling leads to an undesirable exponential increase in subthreshold leakage current.
Reducing the threshold voltage shifts the entire characteristic curve to the left. This shift poses challenges, particularly in ensuring proper device switching off (when Vgs = 0 V), leading to a significant short circuit current. Subthreshold currents associated with this scenario can result in substantial static power dissipation. Subthreshold leakage occurs when carrier diffusion between the source and drain happens after the gate-source voltage (Vgs) has surpassed the weak inversion point but remains below the threshold voltage Vt, where drift becomes dominant.
What does reducing the threshold voltage (Vt) involve, and how is it achieved?
Reducing the threshold voltage (Vt) involves changing the substrate and channel dopant concentrations. This alteration allows the supply voltage to be scaled down without a loss in speed.
What is the drawback of scaling down the supply voltage by reducing the threshold voltage?
Scaling down the supply voltage by reducing the threshold voltage leads to an exponential increase in the subthreshold leakage current, resulting in higher power consumption.
How should the optimum threshold voltage (Vt) be determined for low supply voltage operation?
The optimum threshold voltage (Vt) should be determined based on the current drives at low supply voltage operation and the control of leakage currents. It is essential to balance performance and power consumption.
What causes subthreshold leakage currents, and when do they occur?
Subthreshold leakage currents occur due to carrier diffusion between the source and the drain when the gate-source voltage (Vgs) has exceeded the weak inversion point but is still below the threshold voltage (Vt), where drift is dominant. This leakage current can lead to static power dissipation.